Laser speckle can be caused by interference patterns generated when coherent light illuminates a rough surface. Reflection from the rough surface can create a diffuse reflection. Transmission through the rough surface creates a diffuse transmission. When light hits a rough surface diffuse reflection and/or diffuse transmission occurs and light scatters in various directions. A laser projector uses a coherent laser beam as its light source. When the laser beam from the projector hits the display screen, the light reflects off the surface at various angles and random spatial interference of the coherent laser light with itself occurs. The resulting interference of the coherent light source causes constructive and destructive interference. To the human eye, this image artifact appears as a speckle pattern. These artifacts can cause reduced contrast and resolution of an image in a laser based projection system.
Diffraction can cause another image artifact to occur with laser projectors that use diffractive elements, such as the Grating Light Valve (GLV) modulator described in U.S. Pat. No. 5,982,553. Whereas speckle is caused by interference of the light scattered from the screen, the diffractive elements in the path of a laser beam can cause an interference pattern in the light incident on the screen. This pattern may be visible to the viewer as repeating dark and bright horizontal lines, and it is caused by the interference between diffracted orders after they are recombined.
Several attempts have been made to reduce the effects of laser speckle on image quality. U.S. Pat. No. 3,633,999 discloses the use of a beam splitter to separate a laser beam into two beams. The beams are recombined after the optical path length of one beam has been increased by greater than a coherence length of the laser. Because the two beams are no longer coherent, the re-combined beam can produce less speckle when it is reflected off of a surface such as a screen. However, this method is only effective for lasers that have a short coherence length. An instrument can become difficult to package as the coherence length increases.
U.S. Pat. No. 6,323,984 discloses the use of phase gratings for mitigating the effects of laser speckle on image quality of laser-based projectors that produce a two dimensional image by scanning a line image across a diffuse surface. The phase grating disclosed is a simple two-dimensional pattern consisting of a repeated arrangement of lines of phase delay. A later publication by the inventor [J I Trisnadi, “Speckle-contrast reduction in laser-projection displays,” in Projection Displays VIII, Proc. SPIE 4657, M H Wu, Editor (Soc. Photo-Opt. Instru. Engrs., Bellingham, Wash., 2002), pp. 131-137], describes a more sophisticated configuration using so-called Hadamard matrices to achieve a greater reduction in speckle contrast. The Hadamard matrix phase grating can be vibrated to further reduce the speckle contrast by moving the phase grating relative to the line image for each scan of the projector, where the eye integrates together several sequential scans. However, use of the phase gratings can result in a loss of light when the laser beam is diffracted into higher order diffraction orders with angles that can be too great to be collected by the optics. Further, while the phase grating can reduce speckle contrast, use of phase gratings can also create objectionable interference patterns on the screen. Use of the phase gratings can merely result in trading one image artifact for another.
U.S. Pat. No. 6,738,105 discloses the use of a phase array system to remove the perception of speckle. This patent, however, does not discuss eliminating interference artifacts in a displayed image that are caused by the use of diffractive elements.